Magnetoresistive random access memory cell and fabricating the same

ABSTRACT

The present disclosure provides a semiconductor memory device. The device includes a pinning layer having an anti-ferromagnetic material and disposed over a first electrode; a pinned layer disposed over the pinning layer; a tunneling layer disposed over the pinned layer, a free layer disposed over the tunneling layer and a capping layer disposed over the free layer. The capping layer includes metal-oxide and metal-nitride materials.

BACKGROUND

In integrated circuit (IC) devices, magnetroresistive random access memory (MRAM) is an emerging technology for next generation embedded memory devices. MRAM is a memory device including an array of MRAM cells, each of which stores a bit of data using resistance values, rather than electronic charge. Each MRAM cell includes a magnetic tunnel junction (“MTJ”) cell, the resistance of which can be adjusted to represent logic “0” or logic “1”. The MTJ cell includes an anti-ferromagnetic (“AFM”) pinning layer, a ferromagnetic fixed, or pinned, layer, a thin tunneling layer, and a ferromagnetic free layer. The resistance of the MTJ cell may be adjusted by changing the direction of the magnetic moment of the ferromagnetic free layer with respect to that of the fixed magnetic layer. In particular, when the magnetic moment of the ferromagnetic free layer is parallel to that of the ferromagnetic fixed layer, the resistance of the MTJ cell is low, corresponding to a logic 0, whereas when the magnetic moment of the ferromagnetic free layer is anti-parallel to that of the ferromagnetic fixed layer, the resistance of the MTJ cell is high, corresponding to a logic 1. The MTJ cell is coupled between top and bottom electrodes and an electric current flowing through the MTJ cell from one electrode to the other may be detected to determine the resistance, and therefore the logic state. However, various damages to the MTJ cell, including damage to the free ferromagnetic layer and the tunneling layer during an etching in the fabrication, result in poor performance of the MTJ cell.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIG. 1 is a flowchart of an example method for fabricating a semiconductor memory device constructed according to various aspects of the present disclosure.

FIGS. 2 and 3 are cross-sectional views of an example semiconductor memory device at fabrication stages constructed according to the method of FIG. 1.

DETAILED DESCRIPTION

It is to be understood that the following disclosure provides many different embodiments, or examples, for implementing different features of the disclosure. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. Moreover, the performance of a first process before a second process in the description that follows may include embodiments in which the second process is performed immediately after the first process, and may also include embodiments in which additional processes may be performed between the first and second processes. Various features may be arbitrarily drawn in different scales for the sake of simplicity and clarity. Furthermore, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact.

Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as being “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the exemplary term “below” can encompass both an orientation of above and below. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

FIG. 1 is a flowchart of a method 100 of making a MRAM device constructed according to aspects of the present disclosure. For the sake of example, the method 100 will be described with reference to a MRAM device 200 shown in FIG. 2. The MRAM device 200 is a portion of one non-volatile random access memory (RAM), such as a spin torque transfer magnetroresistive random access memory (STT-MRAM). In an embodiment, the MRAM device 200 includes a stack of material layers designed and configured to function as a magnetic tunnel junction (MTJ).

The MRAM device 200 is formed on a semiconductor substrate, such as silicon substrate or alternatively other suitable semiconductor substrate. The MRAM device 200 includes a first electrode (or bottom electrode) 210 of conductive material. In an embodiment, the first electrode 210 includes titanium (Ti). In other embodiments, the first electrode may include other appropriate materials for forming such an electrode, such as tantalum (Ta), platinum (Pt) or ruthenium (Ru). The first electrode 210 can be formed by a physical vapor deposition (PVD or sputtering) or alternatively other suitable processes.

Referring to FIGS. 1 and 2, the method 100 begins at step 102 by forming a pinning layer 220 over the first electrode 210. In an embodiment, the pining layer 220 includes an anti-ferromagnetic (AFM) material. In the anti-ferromagnetic material layer 220, the magnetic moments of atoms or molecules align in a regular pattern with neighboring spins pointing in opposite directions. In one example, the pinning layer 220 includes platinum manganese (PtMn). In another example, the pinning layer 220 includes iridium manganese (IrMn), rhodium manganese (RhMn) and iron manganese (FeMn). In another example, the pinning layer 220 has a thickness ranging from about 100 angstrom to about 200 angstrom. The pinning layer 220 is formed by a suitable deposition technique, such as PVD.

The method 100 proceeds to step 104 by forming a pinned layer 230 over the pinning layer 220. In an embodiment, the pinned layer 230 includes a ferromagnetic material layer, such as cobalt-iron-boron (CoFeB). The ferromagnetic material layer 230 forms permanent magnets and/or exhibit strong interactions with magnets. Alternatively, the ferromagnetic material layer 230 includes other materials, such as CoFeTa, NiFe, Co, CoFe, CoPt, CoPd, FePt, or the alloy of Ni, Co and Fe. As one example, the CoFeB film is formed by PVD, or alternatively other suitable process.

The pinned layer 230 includes a multilayer structure. In an embodiment, the pinned layer 230 includes a first pinned layer (or bottom pinned layer) 232 and a second pinned layer (or top pinned layer) 236 interposed by a spacer layer 234. In this embodiment, the first pinned layer 232 is disposed on the pining layer 220. In furtherance of the embodiment, the first pinned layer 232 includes a ferromagnetic material. In one example, the ferromagnetic material in the first pinned layer 232 includes a CoFeB film. The ferromagnetic material layer may alternatively include other materials, such as CoFeTa, NiFe, Co, CoFe, CoPt, CoPd, FePt, or the alloy of Ni, Co and Fe.

The spacer layer 234 is disposed on the first pinned layer 232. In an embodiment, the spacer layer 234 includes ruthenium (Ru). Alternatively, the spacer layer 234 may include other suitable material, such as Ti, Ta, Cu, or Ag. The spacer layer 234 may be formed by a PVD process, or other suitable processes.

The second pinned layer 236 is disposed on the spacer layer 234. As an embodiment, the second pinned layer 236 includes a ferromagnetic material layer substantially similar to that of the first pinned layer 230. For example, the second pinned layer 236 includes a CoFeB film. The second pinned layer may be formed by PVD, or other alternative processing method.

In another alternative embodiment, the pinned sub-layer 232 has a first film of a first ferromagnetic material and a second film of a second ferromagnetic material on the first film of the first ferromagnetic material. The spacer sub-layer 234 is formed on the first pinned sub-layer 232 and a second pinned sub-layer 236 is formed on the spacer sub-layer 234. The second pinned sub-layer 236 includes a third film of the first ferromagnetic material and a fourth film of the second ferromagnetic material on the third film of the first ferromagnetic material. In the illustrated embodiment, the pinned layer 230 includes a combination of a CoFeB film (the first film), CoFe (the second film), a Ru layer (the spacer layer), a CoFeB film (the third film) and a CoFe layer (the fourth film) stacked together. In one example, each of the first through the fourth films has a thickness ranging between about 10 angstrom and about 30 angstrom. Each film may have a thickness different from other films of the pinned layer. However, it will be recognized that the pinned layer 230 may be implemented using other types of materials and combinations of materials appropriate for implementing a fixed ferromagnetic layer.

The method 100 proceeds to step 106 by forming a tunneling layer 240 on the pinned layer 230. The tunneling layer 240 includes magnesium (Mg). Alternatively, the tunneling layer 240 may include magnesium oxide (MgO), aluminum oxide (AlO), aluminum nitride (AlN), aluminum oxynitride (AlON), or other suitable material. In another embodiment, the tunneling layer 240 includes a first film of MgO and a second film of Mg over the first film, which may be formed by Mg deposition, Mg oxidation and Mg deposition. The tunneling layer 240 is thin enough such that electrons can tunnel through when a biasing voltage is applied on the MRAM device 200. In an embodiment, the tunneling layer 240 has a thickness ranging between about 5 angstrom and about 15 angstrom. The tunneling layer 240 may be formed by PVD.

The method 100 proceeds to step 108 by forming a free layer 250 on the tunneling layer 240. The free layer 250 may include a ferromagnetic material but it is not pinned because there is no anti-ferromagnetic material adjacent the free layer. Therefore, the magnetic orientation of the layer is free, thus referred to as a free layer. The free layer 250 may include a ferromagnetic material similar to the ferromagnetic material used in the pinned layer 230. In one embodiment, the magnetization direction of the ferromagnetic free layer 250 is free to rotate parallel or anti-parallel to the pinned magnetization of the pinned ferromagnetic layer 230. In an embodiment, the free layer 250 includes a CoFeB film. In one example, the CoFeB may have compositions expressed in (Co_(x)Fe_(1-x))₈₀B₂₀, wherein the x is an alloy mole fraction and ranges between 0 and 100 and the subscripts “80” and “20” are also mole fractions. In another example, the CoFeB may have compositions expressed in Co₂₀Fe₆₀B₂₀. The free layer 250 may be formed by a process, such as PVD. Particularly, the free layer 250 may be formed by the same PVD tool used to form other underlying layers, such as the tunneling layer 240.

The method 100 proceeds to step 110 by forming a capping layer 260 on the free layer 250. The capping layer 260 includes a thin metal-oxide or metal-nitride layer. The metal in the metal-oxide (or metal-nitride) capping layer includes beryllium (Be), magnesium (Mg), aluminium (Al), titanium (Ti), tungsten (W), germanium (Ge), platinum (Pt) and their alloy. The material of the capping layer 260 is chosen such that it has an adequate etching resistance in a subsequent etching which will be described details later. A thickness of the capping layer 260 is chosen such that it provides an adequate protection for the free layer 250 in the subsequent etching and meets a target of tunneling magnetoresistance (TMR) of the MRAM device 200. As an example, the thickness of the capping layer 260 is in a range from about 3 angstrom to about 20 angstrom. The capping layer 260 may be deposited by PVD with an oxygen gas being introduced during the metal deposition to form a metal-oxide capping layer 260. Alternatively, a nitrogen gas is provided during the metal deposition and a metal-nitride capping layer 260 is formed. For an example, a metal film is formed first by sputtering and then converting it into a metal-oxide film by applying oxygen plasma. The concentration of oxygen (or nitrogen) of the capping layer 260 is predetermined by targets of TMR and resistance area (RA) of the MRAM device 200.

The method 100 proceeds to step 112 by forming a second electrode (or top electrode) 270 on the capping layer 260. The top electrode 270 includes a conductive material. The top electrode 270 may be similar to the bottom electrode 210 in terms of composition and deposition. In the depicted embodiment, the top electrode 270 includes Ta. In another embodiment, the top electrode 270 includes TaN. In yet other embodiments, the top electrode 270 may include other appropriate materials, such as titanium (Ti), platinum (Pt) or ruthenium (Ru). The top electrode 270 provides electrical connection of a device to other portions of the interconnect structure for pre-designed electrical routing. The top electrode layer 270 may be formed by a PVD process.

Referring to FIG. 3, a magnetic tunnel junction (MTJ) stack structure 300 is formed in the MRAM device 200 by a patterning process. In an embodiment, a hard mask layer is deposited on the second electrode 270 and is then patterned by lithography and etching processes. By using the patterned hard mask as an etch mask, layers of the top electrode 270, the capping layer 260, the free layer 250, the tunneling layer 240, the pinned layer 230 and the pinning layer 220 are etched (referred to as MTJ stack etching) to form multiple MTJ stack structures 300. The MTJ stack etching includes a dry etch, a wet etch, or a combination of dry etch and wet etch. The dry etching process may implement fluorine-containing gas (e.g., CF4, SF6, CH2F2, CHF3, and/or C2F6), chlorine-containing gas (e.g., Cl2, CHCl3, CCl4, and/or BCl3), bromine-containing gas (e.g., HBr and/or CHBR3), iodine-containing gas, other suitable gases and/or plasmas, and/or combinations thereof. The MTJ stack etching may include a multiple-step etching to gain etch selectivity, flexibility and desired etch profile. The hard mask layer may be removed after the etching. In another embodiment, the hard mask remains on the MTJ stack after the etching.

Based on the above, the present disclosure offers a magnetic tunnel junction (MTJ) stack having the capping layer 260 formed on top of the free layer 250. With the capping layer 260, improvements in critical current density (Jc), TMR and data retention of the MRAM device 200 are observed. It is believed that during the MTJ stack etching, the capping layer 260, equipped with an adequate etching resistance, provides a protection to the free layer 250 and the tunneling layer 240, especially protecting them against edge damage.

The present disclosure provides many different embodiments of a magnetoresistive random access memory (MRAM) cell that provide one or more improvements over other existing approaches. In an embodiment, the MRAM cell includes a pinning layer disposed over a first electrode, a pinned layer disposed over the pinning layer, a tunneling layer disposed over the pinned layer, a free layer disposed over the tunneling layer, a capping layer disposed over the free layer and a second electrode disposed over the capping layer.

Another embodiment is a magnetic tunnel junction (MTJ) stack for a magnetoresistive random access memory (MRAM) cell. The MTJ stack includes a semiconductor substrate, a first electrode formed on the semiconductor substrate, an anti-ferromagnetic layer disposed over the first electrode, a pinned layer disposed over the anti-ferromagnetic layer and the pinned layer having a ferromagnetic material. The MTJ stack also includes a tunneling layer disposed over the pinned layer. The MTJ stack also includes a free layer disposed on the tunneling layer and the free layer having ferromagnetic materials. The MTJ stack also includes a capping layer disposed over the free layer.

In yet another embodiment, a method for fabricating a MTJ stack includes providing a bottom electrode, forming a pinning layer over the bottom electrode, forming a pinned layer over the pinning layer, forming a tunneling layer over the pinned layer, forming a free layer over the tunneling layer, forming a capping layer over the free layer, forming a top electrode over the capping layer. The method also includes forming a magnetic tunnel junction (MTJ) stack structure by patterning and etching the top electrode, the capping layer, the free layer, the tunneling layer, the pinned layer, the pinning layer and the bottom electrode.

It is understood that various different combinations of the above-listed embodiments and steps can be used in various sequences or in parallel, and there is no particular step that is critical or required. Additionally, although the term “electrode” is used herein, it will be recognized that the term includes the concept of an “electrode contact.” Furthermore, features illustrated and discussed above with respect to some embodiments can be combined with features illustrated and discussed above with respect to other embodiments. Accordingly, all such modifications are intended to be included within the scope of this invention.

The foregoing has outlined features of several embodiments. Those of ordinary skill in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those of ordinary skill in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions and alterations herein without departing from the spirit and scope of the present disclosure. 

What is claimed is:
 1. A semiconductor memory device, comprising: a pinning layer over a first electrode; a pinned layer disposed over the pinning layer; a tunneling layer disposed over the pinned layer; a free layer disposed over the tunneling layer; a capping layer disposed over the free layer; and a second electrode disposed over the capping layer.
 2. The device of claim 1, wherein the pinning layer includes anti-ferromagnetic (AFM) materials selected from the group consisting of platinum manganese (PtMn), iridium manganese (IrMn), rhodium manganese (RhMn), and iron manganese (FeMn).
 3. The device of claim 1, wherein the pinned layer includes ferromagnetic materials selected from the group consisting of cobalt-iron-boron (CoFeB), cobalt-iron-tantalum (CoFeTa), nickel iron (NiFe), cobalt (Co), cobalt iron (CoFe), cobalt platinum (CoPt), cobalt palladium (CoPd), iron platinum (FePt), and alloys thereof.
 4. The device of claim 3, wherein the pinned layer includes: a first ferromagnetic sub-layer; a spacer sub-layer disposed on the first ferromagnetic sub-layer; and a second ferromagnetic sub-layer disposed on the spacer sub-layer.
 5. The device of claim 1, wherein the tunneling layer includes materials selected from the group consisting of magnesium (Mg), magnesium oxide (MgO), aluminum oxide (AlO), aluminum nitride (AlN), and aluminum oxynitride (AlON).
 6. The device of claim 1, wherein the free layer includes ferromagnetic materials selected from the group consisting of cobalt-iron-boron (CoFeB), cobalt-iron-tantalum (CoFeTa), nickel iron (NiFe), cobalt (Co), cobalt iron (CoFe), cobalt platinum (CoPt), cobalt palladium (CoPd), iron platinum (FePt), and alloys thereof.
 7. The device of claim 1, wherein the capping layer includes either metal-oxide or metal-nitride materials.
 8. The device of claim 7, wherein a metal of one of the metal-oxide and metal-nitride materials is selected from the group consisting of beryllium (Be), magnesium (Mg), aluminium (Al), titanium (Ti), tungsten (W), germanium (Ge), platinum (Pt), and alloys thereof.
 9. The device of claim 7, wherein a nitrogen concentration of the metal-nitride material is determined by a target of tunneling magnetoresistance (TMR) and resistance area (RA) of the semiconductor memory device.
 10. The device of claim 7, wherein an oxygen concentration of the metal-oxide material is determined by a target of tunneling magnetoresistance (TMR) and resistance area (RA) of the semiconductor memory device.
 11. The device of claim 1, wherein the capping layer is formed with a thickness range from about 3 angstroms to about 20 angstroms.
 12. The device of claim 1, wherein the first and second electrodes each includes materials selected from the group consisting of tantalum (Ta), titanium (Ti), platinum (Pt), ruthenium (Ru), and tantalum nitride (TaN).
 13. A semiconductor memory device, comprising: a semiconductor substrate; a first electrode formed over the semiconductor substrate; an anti-ferromagnetic layer disposed over the first electrode; a pinned layer having a plurality of films disposed over the anti-ferromagnetic layer, the pinned layer having a ferromagnetic material; a tunneling layer disposed over the pinned layer; a free layer disposed over the tunneling layer, the free layer having ferromagnetic materials; and a capping layer disposed over the free layer.
 14. The device of claim 13, wherein the capping layer includes materials containing one of metal-oxide and metal-nitride.
 15. The device of claim 14, wherein the metal of one of metal-oxide and metal nitride is selected from the group consisting of beryllium (Be), magnesium (Mg), aluminium (Al), titanium (Ti), tungsten (W), germanium (Ge), platinum (Pt), and alloys thereof.
 16. The device of claim 14, wherein: a concentration of oxygen of the metal-oxide material is determined by a target of tunneling magnetoresistance (TMR) and resistance area (RA) of the semiconductor memory device; and a concentration of nitrogen of the metal-nitride material is determined by the target of tunneling magnetoresistance (TMR) and resistance area (RA) of the semiconductor memory device.
 17. A method of fabricating a magnetroresistive random access memory (MRAM) cell, comprising: forming a pinning layer over a bottom electrode; forming a pinned layer over the pinning layer; forming a tunneling layer over the pinned layer; forming a free layer over the tunneling layer; forming a capping layer over the free layer; forming a top electrode over the capping layer; and forming a magnetic tunnel junction (MTJ) stack structure by patterning and etching the top electrode, the capping layer, the free layer, the tunneling layer, the pinned layer, the pinning layer, and the bottom electrode.
 18. The method of claim 17, wherein forming the capping layer includes performing a deposition in a physical vapor deposition (PVD) tool, the method further comprising: introducing oxygen during the deposition to form a metal-oxide capping layer; and adjusting an oxygen concentration of the metal-oxide capping layer according to a target of tunneling magnetoresistance (TMR) and resistance area (RA) of the magnetroresistive random access memory (MRAM) cell.
 19. The method of claim 17, wherein forming the capping layer includes performing a deposition in a physical vapor deposition (PVD) tool, the method further comprising: introducing nitrogen during the deposition to form a metal-nitride capping layer; and adjusting a nitrogen concentration of the metal-nitride capping layer according to a target of tunneling magnetoresistance (TMR) and resistance area (RA) of the magnetroresistive random access memory (MRAM) cell.
 20. The method of claim 18, wherein the metal in the metal-oxide capping layer includes materials selected from the group consisting of beryllium (Be), magnesium (Mg), aluminium (Al), titanium (Ti), tungsten (W), germanium (Ge), platinum (Pt), and alloys thereof. 